1. Field of the Invention
The present invention relates to a composite material including an organic compound and an inorganic compound, a light-emitting element, a light-emitting device, an electronic device, and a lighting device.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using organic electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting organic compound is interposed between a pair of electrodes. By application of a voltage to this element, light emission from the light-emitting organic compound can be obtained.
Since such light-emitting elements are of self-light-emitting type, it is considered that the light-emitting elements have advantages over liquid crystal displays in that visibility of pixels is high, backlights are not required, and so on and are therefore suitable as flat panel display elements. In addition, it is also a great advantage that the light-emitting elements can be manufactured as thin and lightweight elements. Furthermore, very high speed response is also one of the features of such elements.
Furthermore, since such light-emitting elements can be formed in a film form, they make it possible to easily form large-area elements. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, light-emitting elements also have great potential as planar light sources applicable to lighting devices and the like.
As described above, application of light-emitting elements using organic EL to light-emitting devices, lighting devices, or the like is expected. At the same time, there are many issues regarding light-emitting elements using organic EL. One of the issues is a reduction in power consumption. In order to reduce power consumption, it is important to reduce driving voltage for the light-emitting element. Further, in order to reduce the driving voltage, it is necessary to feed a large amount of current at a low voltage because the emission intensity of the light-emitting element using organic EL is determined by the amount of electric current flowing therein.
Previously, as a method for reducing driving voltage, an approach of providing a buffer layer between an electrode and the layer containing a light-emitting organic compound, has been attempted. For example, it is known that driving voltage can be reduced by providing a buffer layer which includes polyaniline (PANI) doped with camphorsulfonic acid, between indium tin oxide (ITO) and a light-emitting layer (see Non-Patent Document 1, for example). It is explained that this is because PANI has a property of excellent carrier injection into the light-emitting layer. Note that in Non-Patent Document 1, PANI, which is used for the buffer layer, is also regarded as part of the electrode.
However, as described in Non-Patent Document 1, PANI has a problem that transmittance becomes poor when a film thickness becomes thick. Specifically, it is reported that at a film thickness of about 250 nm, the transmittance is less than 70%. In other words, since the problem lies in the transparency of the material itself used for the buffer layer, light generated within the element cannot be extracted efficiently.
Also, according to Patent Document 1, an approach of serially connecting light-emitting elements (called light-emitting units in Patent Document 1) to improve luminance per a certain current density, namely, current efficiency, has been attempted. In Patent Document 1, for a connecting portion of serially connected light-emitting elements, a mixed layer of an organic compound and a metal oxide (specifically, vanadium oxide or rhenium oxide) is used, and this layer is considered capable of injecting holes and electrons into light-emitting units.
However, as apparent by looking at an embodiment, for the mixed layer of an organic compound and a metal oxide which is disclosed in Patent Document 1, not only a high absorption peak in the infrared region but also a high absorption peak in the visible light region (around 500 nm) are observed, and a problem in transparency occurs. This is due to the effect of an absorption band generated by charge-transfer interaction. Therefore, as expected, light generated within the element cannot be extracted efficiently, and the light emission efficiency of the element is degraded.